Iron-overload due to transfusion currently occurs with any patient who receives more than 30 or 40 transfusions over the course of his or her life. The excess iron can injure any organ in the body. The heart and liver are particularly susceptible to damage, and failure of one of these organs is often the cause of death in patients with transfusional iron-overload. Transfusion related iron-overload is a major cause of morbidity and mortality in patients with a variety of transfusion-dependent anemias, hereditary hemochromatosis, including thalassemia major (Cooley's anemia).
Transfusion associated iron-overload develops in conditions characterized by severe, life-threatening anemia where transfusions substantially prolong life expectancy (Cairo, M. (1990) Introduction. Am. J. Pediatr. Hematol. Onco. 12:1-3). The most notable causes of transfusional iron-overload are the thalassemias, mild aplastic anemia, and congenital anemia (McLaren G. M. W. et al. (1983) CRC Crit. Rev. Clin. Lab. Sci. 126:896-899). Children with sickle cell disease and children who are stroke prone with sickle cell disease who receive chronic transfusions for complications may also suffer from iron-overload (Pegelow, C. et al. (1997) J. Pediatr. 126:896-899; Adams, R., et al. (1988) NEJM 339:5-11).
Transfused red cells are engulfed and destroyed at the end of their life span by stationary reticuloendothelial cells in the liver (Kupfer cells) and the spleen. The iron from the hemoglobin is removed and stored as hemosiderin. The reticuloendothelial cells return some of this iron to the circulation coupled to transferrin, and the iron is redistributed to the cells of the body. No known physiological mechanism of iron excretion exists. Therefore, after a number of transfusions, the level of iron in the body reaches a toxic level. At that point, chelation therapy is required. Iron that is stored in hemosiderin is innocuous. This iron is in equilibrium, however, with a very small pool of so-called “free iron” in the cell. This pool of iron is so small that its size has never been satisfactorily determined. Better termed “loosely-bound iron,” this material catalyzes the formation of reactive oxygen species through Fenton chemistry. These reactive oxygen species are the agents of cell injury.
Iron-overload, whether due to chronic transfusions or hereditary hemochromotosis, has a plethora of side-effects (Bonkovsky, H. (1991) American Journal of Medical Science 301:32-43), (Koren, A. et al. (1987) Am. J. Dis. Child. 141:93-96). Liver damage and heart failure are the two most common causes of death. Liver iron deposition initiates hepatic fibrosis, cirrhosis and death (Bonkovsky, H. (1991) American Journal of Medical Science 301:32-43). Congestive heart failure or death from cardiac arrhythmias is common (Koren, A. et al. (1987) Am. J. Dis. Child. 141:93-96). For disorders such as thalassemia major (by definition, transfusion-dependent thalassemia), iron-overload now is the limiting factor in survival. The advent of chronic transfusion therapy in the 1960's increased life span to the early to mid-twenties, however, the pernicious consequences of iron-overload were invariably fatal (Cooley, T. (1945) Am. J. Med. 209:561-572), (Piomelli, S. (1991) Hematol. Oncol. Clin. North. Am. 5:557-569).
Iron is one of the leading causes of pediatric poisoning deaths in the United States (Litovitz, T. L. et al. (1992) Am. J. Emerg. Med. 10:452-505). Numerous reports of serious or fatal poisonings have been cited in the medical literature (Litovitz, T. L. et al. (1992); Westlin, W. F. (1966) Clin. Pediatr. 5:531-535; Henriksson, P. et al. (1979) Scand. J. Haematol. 22:235-240) including five toddler deaths in Los Angeles county during a seven month period in 1992 (Weiss, B. et al. (1993) Morb. Mortal. Wkly. Rep. 42:111-113). It is clear that iron can cause serious morbidity and mortality, yet many clinicians and families remain unaware of the dangers of iron (Anderson, B. D. (Apr. 18, 2000) Medscape Pharmacists). Although uncommon, iron solutions may be absorbed through damaged or burned skin. Following ingestion of large amounts of iron, peak serum levels generally occur within 2 to 6 hours. After ingestion, iron in the +2 state is oxidized to the +3 state and attached to the transport protein, ferritin. The iron is then released from the ferritin to transferrin in the plasma, transported to the blood forming storage sites, and incorporated into enzymes in the body. Iron is eliminated slowly from the body. Even in states of iron overload, children may lose up to 2 mg per day. Ingestion of less than 20 mg/kg elemental iron is likely to produce GI symptoms. For patients who ingest greater than 60 mg/kg elemental iron, potentially life threatening symptoms may occur.
Furthermore, the emergence of drug resistant parasites, e.g., malaria, has intensified the search for new therapeutic approaches (e.g. drug combinations). One new approach under investigation is the administration of iron chelating agents (Cabantchik, Z. I. et al. (1996) Acta Haematol. 95:70-77; Van Zyl, R. L. et al. (1992) J. Antimicrob. Chemother. 30:273-278).
Patients with transfusion iron-overload, iron poisoning, and drug resistant parasitic diseases (e.g., malaria) are commonly treated with low molecular weight iron chelators. These compounds remove the excess, toxic iron from the patient's blood. The most commonly used drug worldwide is desferrioxamine (Desferal®, Novartis). Therapy with desferrioxamine is effective, but under-utilized because of drug delivery problems. Oral absorption of desferrioxamine is very low. In some cases, desferrioxamine infusion has proven not to be adequate (Westlin, W. (1996) Clin. Ped. 5:531-535; Tenenbein, M. et al. (1992) Lancet 339:699-701; Adamson, I. Y. et al. (1993) Toxicol. Appl. Pharmacol. 120:13-19). In addition, the low molecular weight of this hydrophilic molecule (657 Da) leads to renal clearance in about 15 to 20 minutes. Consequently, the drug is given by continuous infusion over 12 to 16 hours. This is done either by subcutaneous infusion or by infusion into a permanent catheter. Such a long infusion duration is inconvenient and prone to infections and thrombosis. Desferrioxamine also has severe drawbacks in the treatment of parasitic diseases; (1) it is hydrophilic and poorly absorbed after oral administration; and (2) it is cleared rapidly after intravenous administration and iron chelators like desferrioxamine do not readily penetrate into advanced growth stages of parasitized cells (Loyevsky, M. et al. (1993) J. Clin. Invest. 91:218-224). As a consequence, continuous infusion of iron chelators like desferrioxamine over a three day period is required to obtain enhanced parasite clearance in human malaria (Mabeza, G. F. et al. (1996) Acta Haematol. 95:78-86). Nonetheless, many patients use desferrioxamine suboptimally or not at all.
No other chelator has proven clinical efficacy. Searches for clinically effective alternatives to desferrioxamine for transfusional iron-overload have thus far been futile. Some chelating agents, such as diethyltriamine pentaacetic acid (DPTA) are effective, but too toxic for clinical use. Other chelators (e.g., EDTA) bind other cations in addition to iron, making them unacceptable as treatment of transfusional iron-overload or iron poisoning.
One approach to the problem has been to immobilize desferrioxamine to a large molecular matrix, thereby extending its biological half life. Immobilized desferrioxamine depends on a shift in “pseudoequilibrium” conditions to produce a net outflux of iron from cells. The vast amount of storage iron exists inside cells, however, effectively out of the reach of immobilized desferrioxamine. The problem is that the storage iron inside the cells remains a dangerous source of free radicals until it is chelated and inactivated by the Desferrioxamine in the matrix.
U.S. Pat. No. 5,534,241 ('241 patent) discloses chelation of iron using a linked molecule having a polymeric moiety covalently bonded to a lipid soluble anchor and a plurality of chelating agents covalently bonded to the polymeric moiety. There are several drawbacks to the compound disclosed in the '241 patent. In particular, complicated chemical synthesis and purification are required to covalently link the various groups. Also, to prevent rapid degradation of the '241 patent compound, surface protection is required. In addition, the polychelating agent is not free but is instead bound to the polymeric moiety, which can limit its ability to chelate iron from cellular stores.
The only chelator currently in extensive clinical trial is deferiprone (L1). Deferiprone removes excess iron reasonably well although it falls short of desferrioxamine in this regard (Collins, A. et al. (1994) Blood 83:2329-33). The great appeal of deferiprone over desferrioxamine is its oral absorption. For many patients, the convenience of an orally active chelator might more than compensate for lesser efficacy.
A number of clinical problems cloud deferiprone's future. Severe agranulocytosis occurs in about 2% of patients (al-Refaie, F. et al. (1992) Blood 80:593-9). Other significant side-effects of deferiprone include arthralgias and severe nausea (al-Refaie, F. et al. (1995) Br. J. Haematol. 91:224-229). Because of these and other problems, deferiprone's clinical future is far from assured. Therefore, there exists a need for an improved iron chelator delivery system to remove iron-overload in a cell, tissue, or organ.